Edgard Garrido / Reuters

If immigrants, including masses of “illegal aliens” from Mexico, are so damned dangerous, why isn’t Trump’s hometown of New York City the scariest hellhole in the country? Why is it, in fact, one of the safest big cities in the United States?

After all, almost 40 percent of the Big Apple’s population was not born in the USA. The Mexican community, particularly, has grown tremendously over the last two decades, from a few tens of thousands to more than 300,000, according to census figures, and 500,000 by some unofficial estimates. Yet violent crime statistics, particularly homicides, have declined to record lows.

The fact is, first-generation immigrants, including Mexican indocumentados, help to make big cities safer.

People responsible for maintaining law and order have understood this for years. Rudy Giuliani, New York’s “zero-tolerance” mayor of the 1990s, used to declare, “If you come here and work hard and you happen to be in an undocumented status, you’re one of the people we want in this city.” And he set the “safe haven” precedent that people using New York City services—including and especially when they called the police—would not be referred to federal immigration officers.

Former NYPD Commissioner Ray Kelly told me flatly in an email this week that immigration is “the strength of the city.”

Trump, whose own fortune has depended so much on Gotham’s, could not help but be aware of this. And that’s why even as he talks about stopping Mexico from supposedly sending its criminals north, and blusters on about building walls along the Rio Grande, he also talks vaguely about opening the way for people without papers to sort out their status.

“I’m a very big believer in the merit system,” Trump said on his hyper-publicized visit to the Mexican border last month. “I have to tell you: some of these people have been here, they’ve done a good job. You know, in some cases, sadly, they’ve been living under the shadows.… If somebody’s been outstanding, we try and work something out.”

Right. When we’re talking about more than 11 million nationwide living under the shadows, trying to “work something out” for “some of these people” doesn’t really cut it. But at least Trump’s pointing in the right direction, which is more than one can say about the congressional efforts to force local officials to enforce federal immigration laws in so-called “sanctuary cities.”

The popular conception that first-generation immigrants are a major source of crime rather than a major source of security is, ironically, deeply ingrained in our nation-of-immigrants DNA. It was at the heart of the appropriately named “Know-Nothing” party in the 1800s and it has driven polls over the last 20 years that have shown consistently 60 to 75 percent of the population believes more immigrants cause higher crime rates. As the old joke goes: “My family has been having trouble with immigrants ever since they came to this country.”

Every so often, racism and xenophobia gain added traction from gruesome anecdotes. In 2010, for instance, the murder of rancher Robert Krentz in the Arizona desert—still unsolved—was used to push through that state’s controversial SB 1070 law which tried to compel local police to waste their time enforcing federal immigration laws. More recently, hysteria has been fueled by the killing of young Kate Steinle in San Francisco. The alleged murderer is a Mexican scumbag who’d been in and out of American jails for years, been deported, come back, and learned too well how to exploit loopholes in the system, including one judge’s interpretation of San Francisco’s “sanctuary” rules, to walk free before this latest crime.

The high-profile activities of gangs with ties to the savage drug cartels of Mexico or the ferocious maras tearing Central America apart raise legitimate concerns. But the gangs, often made up of young men and women citizens born in the USA and indoctrinated by the American prison system, certainly are no more representative of the larger immigrant communities of today than Meyer Lansky’s mob was representative of Jews, Al Capone’s of Italians or Whitey Bulger’s of Irish-Americans.

“Almost everyone who has examined the issue and is not an ideologue has come to the same conclusion,” criminologist James Lynch told me in 2007. “In the United States, immigrants engage in common law crime at rates lower than the native population.” And Lynch, now head of the criminology and criminal justice department at the University of Maryland, says he stands by that statement today.

But once the melting pot is stirred up with paranoid tales of terror, the actions of a few can always be used to condemn the many, and those who present evidence to the contrary quickly become the target of vitriolic attacks.

Robert J. Sampson, a sociologist at Harvard, has been writing for years about the correlation between large first-generation immigrant populations and lower crime rates, and, as he points out in a new article in American Prospect, he’s found himself the target not only of “the anger and bile that is typical online, but also assertions that the idea was crazy—‘lunacy,’ as one response put it.”

Sampson readily concedes that “correlation does not prove causation,” but a growing body of data, and the experience of New York City, certainly, shows that lower crime rates can be linked to several benefits brought by first-generation immigrants.

As I wrote in Securing the Cityin 2009, “it’s not just that newly arrived immigrants are less likely to be part of an urban nightmare, it’s that they bring their own positive dreams.” Marginal neighborhoods, instead of becoming empty urban wastelands, fill up with new immigrants. They are a self-selected group of people with energy and initiative whose aim—like that of every generation of immigrants since before the creation of the nation—is to build a future for themselves and their families. Fear of deportation may make them particularly careful about breaking the law, but their main goal is to work and to save, and as they build their futures they build the economy and they enhance security

If there is a downside, it’s this: the second and third generation, as they become more American, often become more troubled and troubling. Their immigrant backgrounds make it harder for them to integrate into U.S. society even as they lose their roots in the lands of their fathers. They may pick up the vices of American society, from obesity to violence, without a chance to share in the virtues. And for a few—whether their parents are Salvadoran, Mexican, Hmong, or Somali—gangs do become a means of asserting their identity. When that happens, violent crime may increase.

But what does that tell us? The challenge for society, and the one today’s Know-Nothings should address (but won’t), is not so much keeping immigrants out, it is keeping their dreams alive for their children and grandchildren.